It is known that, in theory, individual, load-control port throttles (referred to herein as classical) can achieve fast torque response to driver accelerator pedal demand, reduce pumping work at light-medium engine loads and deliver improved combustion stability at light loads with high overlap valve timing compared to plenum throttling.
Furthermore, it is known that, in order to satisfy varying load demands (and therefore, varying engine air flow) at engine idle, current mass-production internal combustion engines require modulation of the idle air flow by some means--usually an idle speed control valve with an actuating solenoid managed by an Engine Control computer. Consequently, an engine with load-control port throttles preferably also has an idle speed control system. This is typically accomplished by using a small idle manifold connected to each of the intake ports downstream of the port throttles. (This will generally be referred to herein as conventional port throttling.) This manifold receives air from an idle speed control valve which regulates the quantity of air for idle operation. However, because the individual intake ports are interconnected with each other through this idle manifold, an average level of intake vacuum is maintained in the idle manifold as the result of each cylinder's being on a different part the 4-stroke cycle. This allows for cross-communication between the cylinders, which is undesirable.
For a light load engine operating condition, there are differences in the cylinder pressure versus volume, between the classical and the conventional port throttling cases. Classical port throttling at light loads achieves full intake port pressure recovery during the intake valve closed period, so when the intake valve opens, the pressure in the port is essentially atmospheric. This atmospheric pressure results in reduced pumping work during the first part of the intake stroke and will also prevent exhaust back flow during the valve overlap period, reducing exhaust residual fraction and improving the combustion stability of the engine at idle. Conventional port throttling that uses an idle manifold, with the interconnection of the intake ports through the idle manifold, reduces the intake port pressure at the time the intake valve opens, which increases the pumping work and provides a significant increase in the exhaust gas back flow, thereby increasing exhaust residual fraction, and thereby reducing combustion stability compared to the classical port throttling case.
A desire exists then, to have a port throttling type of air intake system that operates like a classical throttle, while still incorporating an idle speed control system, without the drawbacks of conventional port throttling.